An R--Fe--B permanent magnet is a representative of the high performance permanent magnets known at present. The excellent magnetic characteristics of an R--Fe--B permanent magnet as disclosed in JP-A-59-46008 (the term "JP-A-" as used herein signifies "an unexamined published Japanese patent application") is attributed to the composition comprising a tetragonal ternary compound as the principal phase and an R-rich phase. The R--Fe--B permanent magnet above yields an extraordinary high performance, i.e., a coercive force iHc of 25 kOe or higher and a maximum energy product (BH)max of 45 MGOe or higher, as compared with the conventional high performance rare earth-cobalt based magnets. Furthermore, a variety of R--Fe--B based permanent magnets varied in composition are proposed to meet each of the particular demands.
To fabricate various types of R--Fe--B based permanent magnets as mentioned hereinbefore, an alloy powder having a predetermined composition should be prepared at first. The alloy powder can be prepared by an ingot-making and crushing process as disclosed in JP-A-60-63304 and JP-A-119701, which comprises melting the starting rare earth metal materials having subjected to electrolytic reduction, casting the melt in a casting mould to obtain an alloy ingot of a desired magnet composition, and then crushing the ingot into an alloy powder having the desired granularity. Otherwise, it can be prepared by a direct reduction diffusion process as disclosed in JP-A59-21940 and JP-A-60-77943, which comprises directly preparing an alloy powder having the composition of the desired magnet from the starting materials such as rare earth metal oxides, iron powder and Fe--B alloy powder.
The ingot-making and crushing process involves many steps, and, moreover, it suffers segregation of an R-rich phase and crystallization of iron (Fe) primary crystals at the step of casting the alloy ingot. According to this process, however, an alloy powder containing relatively low oxygen can be obtained, since the ingot can easily be prevented from being oxidized in a coarse grinding (primary crushing).
The direct reduction diffusion process, on the other hand, is advantageous as compared with the ingot-making and crushing process above in that the steps such as melting and coarse grinding can be omitted from the process of preparing the starting alloy powder for the magnet. However, as compared to the R-rich phases in the former process, the R-rich phases being formed by this process are smaller and well dispersed, and are mostly developed at the surroundings of the principal R.sub.2 Fe.sub.14 B phase. The R-rich phase thus formed in this process is susceptible to oxidation, which, as a result, takes up a considerable amount of oxygen. In some kinds of magnet composition, the rare earth metal elements may be oxidized and consumed by the excess oxygen, and an unstable magnet characteristics may result therefrom.
It can be seen that the oxygen incorporated in the alloy powder deteriorates the magnet characteristics of an R--Fe--B permanent magnet. Accordingly, with an aim to reduce the content of the unfavorable oxygen of the alloy powder, the present inventors have proposed previously, as disclosed in Japanese patent application No. 02-229685, a process which comprises first preparing an alloy powder having a composition near to that of the R.sub.2 Fe.sub.14 B phase by direct reduction diffusion process, while preparing separately a powder of intermetallic compounds such as an R.sub.2 (Fe,Co).sub.17 phase containing an R.sub.3 Co phase (in which iron (Fe) may be present as a substitute for a part or a large part of Co) by adding metallic cobalt into the R-rich alloy powder, and then mixing them both to obtain an alloy material powder for an R--Fe--B permanent magnet.
The proposal above is extremely effective for reducing the oxygen content of the magnet and the starting powder material for an R--Fe--B permanent magnet, however, not only the principal R.sub.2 Fe.sub.14 B phase but an R-rich phase and a B-rich phase, which are known also to deteriorate the intrinsic properties, remain in the magnet. It has been found extremely difficult to control precisely the content of these phases, and hence, these phases remain as the cause for destabilizing the magnetic characteristics.